Effects of Skewing Bench Planes

[Jim Koepke]

Just curious. Been planing some wood and recall once reading that skewing a bench plane lowers the effective cutting angle of the blade.

Does anyone here know the math involved in determining the effective angle based on the amount of skew?

Are there any other effects of skewing a plane?

Jim

[Pedro Reyes]

The cosine of the skew angle times your bed angle? (where 0 degrees is no skew and 90 is perfectly perpendicular to planing direction)

I think, my math is rusty and I am at work so no time to run the checks, but pretty sure, I can check later.

/p

[Jim Koepke]

The cosine of the skew angle times your bed angle? (where 0 degrees is no skew and 90 is perfectly perpendicular to planing direction)

I think, my math is rusty and I am at work so no time to run the checks, but pretty sure, I can check later.

/p

If this is correct, then a skewing of 60 degrees would give an effective work angle of 22.5 degrees.
So, with a fat plane like a 4-1/2, one may already have a LA bench plane.

Can hardly wait for your checks.

Jim

[Wiley Horne]

Try this one,

Tan (effective angle) = cos (skew angle) x tan (bedding angle)

Wiley

[Mike Cutler]

From the Bridge City tool site.

[Pedro Reyes]

This bugs me.

Wiley is right. I am trying to arrive at it using trig. But too busy at work. FWIW at a 45 degree angle, the simple method is pretty close, at higher angles it becomes bad. My first stab is in blue Wiley's formula in yellow.


81456

At the bottom you have your skew angle and your effective angle is on the vertical axis.

/p

[Cliff Rohrabacher]

I would say it differently.
I'd say that it changes the attack of the edge to the fibers along a different angle than the angle of the blade or grind.

There are two angles. One is the angle at which the blade's edge engages the work in the conventional 90-Deg position. That'll be the 22.5 or whatever you have between the grind and how far the plane lays the blade over.
You can't change those without regrinding the blade.

Then there is that 90 degrees at which the blade normally approaches the fibers and grain. That angle you can change by changing the angle of attack. It serves to enhance the slicing action from a chop to a slice taking more time and blade length to go through any given fiber. It's less forceful and that's why it works better on difficult grain.

Interestingly changing the angle of attack requires some more effort because you have the blade engaged with each fiber both along more of the blade and longer in time.

[Mark Stutz]

WOW! Trig...cosine...sine...tangent...still makes my head swim, even after 30 years:eek::confused: Barely survived that course.

Mark

[Steve Rozmiarek]

WOW! Trig...cosine...sine...tangent...still makes my head swim, even after 30 years:eek::confused: Barely survived that course.

Mark

Hey Mark, lets see if they get this one figured out without us all having trig flashbacks. #140 skewed nose to the right, now to the left!:D

[David Bodkin]

Well, at least there is some way I can contribute.

Take a look at the diagram.
Red=bedding angle
Blue=skew angle
Green=effective angle

Both "a" are the same length.
Define some trig stuff
(1) tan(red)=a/b
(2) cos(blue)=b/c
(3) tan(green)=a/c

rewriting..(2) and (3) b = c*cos(blue) a=c*tan(green)

combine (1) and the rewritten (2) and (3)

tan(red)=[c*tan(green)]/[c*cos(blue)]

The "c" cancel out and you can rewrite it..

tan(green)=tan(red)*cos(blue)

Which is the same as Wiley's eq..
Tan (effective angle) = cos (skew angle) x tan (bedding angle)